The APC BackUPS Pro 1500VA UPS, model BR1500G, has an external battery port on its back. This port is interfaces with the BP24BPG, which is a 24V external battery pack holding four 9Ah or 7.5Ah 12V sealed lead-acid (SLA) batteries. These batteries are identical to the two which are in the UPS, itself, but in 2x2 series-parallel configuration, giving 24V.

The external battery pack looks like, is the same size, and costs just as much as the UPS: $180 new from APC and most retailers.

For anyone who works with the APC SC-series UPSes and has replaced those batteries, this port will look familiar. It is physically compatible with the plug on the RBC-33, which is two batteries strapped together in series using the 870-2183(a) harness. In fact, it is the identical plug used to connect the BP24BPG external battery pack to the BR1500G. However, plugging the RBC-33 into the external battery port does not give any results as the UPS simply ignores the battery pack.

Examination of both the port and the plug reveals three connectors. Obviously two are for the positive and negative terminals of the batteries. As both the BP24BPG and RBC-33 are 24V packs, it was fair to consider the third connection could be a "sense" line with some value of resistance to indicate the unit's purpose.

Taking a look at the 870-2183 harness proves this idea out, showing the heavy gauge black and red wires, and smaller gauge yellow wire coming out of the cable into the plastic bracket which holds the two batteries together. There is also a short heavy gauge yellow wire which jumpers the negative and positive terminals of the batteries. This configuration puts the batteries in series for 24V output.

Fortunately, I did not just jump on the idea of the sense line being some level of resistance. It terminates directly on the positive connector, putting a full 24V down that line, which could have blown my ohm meter right out. Here you can see the yellow wire routed on one side of the plastic bezel of the RBC-33, then on the other side where it is crimped right into the positive connector.

At the plug, the wires in the cable match up to the three "prongs" exactly as they run in the cable. This plug is marked with each connector's polarity. The multi-meter confirms there is no resistance between the plug end of the yellow wire and where it terminates at the positive battery terminal. But this still does not answer the question of, why does the UPS not recognize this as an external battery pack? What does the BP24BPG do differently? It cost me $160 plus shipping to find out. Following the same reverse-engineering procedures on this pack as on the RBC-33, I found the connector uses the same polarity and the sense line is grounded instead of connected to the 24V positive.

This is an easy enough modification to make: simply move the sense wire to the negative terminal on the RBC-33 battery pack's 870-2183 battery harness. Once connected to UPS, the UPS makes a "beep" to indicate it recognizes the external battery.

APC advertises the battery pack as tripling the run-time of the UPS. This makes sense as it does triple the number of batteries connected to the unit: four external and two internal. Using the modified RBC-33 only adds two external batteries, which should give double the run time. This is bore out in testing by changing the external pack in use on a UPS under load. While running my chest freezer, the UPS shows 114 minutes of run-time available. With the BR24BPG connected that run time increases to 338 minutes, and with the modified RBC-33 connected the UPS reports 232 minutes.

I had concerns about how the UPS would behave charging just two external batteries rather than the proper four, so I ran the unit supervised for three days in this new configuration. I found it does properly manage the two external batteries.

Though the connecting cable is shorter, this modification is a viable alternative to much more bulky full battery pack. The harness can be found inexpensively on its own from various sources, including some local battery stores, making it economical and perfectly usable. Even buying a new RBC-33 is half the price of the BP24BPG, commensurate with the extra run time provided versus the full pack. I have several of the harnesses around, and this low-cost alternative will be very handy during the next extended power outage from a storm or hurricane.

UPDATE: At the time I posted this, the UPS in question had been running for two weeks with the modified RBC-33 attached. Today it started running its fan constantly. The only relief is to remove the battery pack. Not certain what is causing that, or if the behavior is isolated to this UPS. It could be the self-test is detecting the unauthorized pack. I will have to test out another UPS and see if it duplicates this behavior. If so, I might be able to get it to "tell" me what is wrong.

The fan did eventually stop. Again, not sure why it started up, but it seems perhaps it was giving a charge to the battery. I noticed the UPS with the BP24 acted similarly after having run on battery for a minute. The modified RBC-33 still works out to be a good configuration.